High voltage bushings are used for carrying current at high potential through a plane, often referred to as a grounded plane, where the plane is at a different potential than the current path. High voltage bushings are designed to electrically insulate a high voltage conductor, located inside the bushing, from the grounded plane. The grounded plane can for example be a transformer tank or a wall.
In order to obtain a smoothening of the electrical potential distribution between the conductor and the grounded plane, a bushing often comprises a number of floating, coaxial foils made of a conducting material and coaxially surrounding the high voltage conductor, the coaxial foils forming a so called condenser core. The foils could for example be made of aluminium, and are typically separated by a dielectric insulating material, such as for example oil impregnated or resin impregnated paper. The coaxial foils serve to smoothen the electric field distribution between the outside of the bushing and the inner high voltage conductor, thus reducing the local field enhancement. The coaxial foils help to form a more homogeneous electric field, and thereby reduce the risk for electric breakdown and subsequent thermal damage.
Such coaxial foils typically provide efficient capacitive grading of the electric field within the bushing. However, a local field enhancement in the vicinity of the foil edges typically remains. The enhanced field at the foil edges limits the operational voltage that can be applied between the high voltage conductor and the grounded plane.
Efforts to grade the electric field at the foil edges of a bushing condenser core are disclosed in U.S. Pat. No. 4,370,514. Here, double layer foils containing an electrically conducting layer and an insulating layer are coaxially arranged around a high voltage conductor, where the insulating layer has a high dielectric constant. At the foil edges, the double layer foils are folded so that the insulating layer encloses the electrically conducting layer in order to improve the ability of the bushing to withstand partial corona discharges and surge voltages. U.S. Pat. No. 4,370,514 also discusses the possibility of limiting the field stress around the foil edges by terminating the foils with a bead-like enlargement, in order to obtain a radius of curvature at the edge which is as large as possible.
The techniques for reducing the field stress at the foil edges discussed in U.S. Pat. No. 4,370,514 increase the radius of the condenser core, and therefore the radius of the bushing. As the electric power technology advances, higher voltages can be employed in various applications and bushings which may withstand higher potentials are therefore required. At the same time, the physical space available to a bushing is typically limited. Therefore, it is desired to find bushings that have an improved relationship between voltage-withstanding properties and bushing diameter.